Although glass capillary and open tubular columns had been used in gas chromatography for some time, their usefulness was limited by their fragility and chemical reactivity. Because of surface hydroxyls, characteristically present on glass surfaces, the peak shapes of reactive materials such as phenols, amines, and mercaptans inter alia tended to be distorted even when the glass surface was deactivated. Thus, in the "early" gas chromatographic days of capillary and open tubular columns the many advantages of glass capillary and open tubular columns were accompanied by serious disadvantages.
In about 1980 the situation changed remarkably based on art taught by U.S. Pat. No. 4,293,415. The '415 patent taught that silica capillary columns for gas chromatography could be made in much the same manner as hollow optical fibers, e.g., liquid core quartz fibers. After treatment of the silica surface with deactivating agents the same as, or analogous to, those used for glass columns, more symmetrical peaks were obtained in gas chromatography and the patentee's silica columns even could be used for classes of compounds for which glass columns were unsatisfactory, as for example phenols, volatile carboxylic acids, mercaptans, and aliphatic amines.
U.S. Pat. No. 5,552,042 discloses a silica capillary or open tubular assembly wound on and fused to a mandrel, with the entire assembly being annealed so as to present a relatively stress free, rigid winding. The relative lack of stress eliminated the need for any external coating. The rigidity of the assembly allowed the deposit of brittle coatings, especially of inorganic materials, within the capillary or tubes as a stationary phase and usefully employed over extended time periods.
In the present state of the art a capillary column can have a larger sample capacity by increasing its bore and applying a thicker stationary phase or it can have higher plate efficiency by reducing its bore diameter and using a thinner stationary phase. A way of getting both benefits simultaneously is to use a multi-capillary. A multi-capillary is a grouping of many capillaries all bundled together. This results in a Van Deemter curve that is relatively flat compared to conventional single capillary columns. The benefits of a multi-capillary have been known for several decades but such has not been commercially implemented because of the difficulties in manufacturing a multi-capillary using conventional glass capillary drawing techniques.
Image intensifiers, also known as night vision scopes, use a structure comprising bundles of fibers. The method for manufacturing the fiber bundle for a night vision scope draws a circular glass tube through a hex die. The resultant hex shaped tubes with circular holes are then grouped together and drawn together as an assembly to form a fused bundle of reduced cross-section. The external hex shape of individual capillaries facilitates the packing together of a grouping of capillaries., i.e. similar to a bee's honeycomb.
It is known in the art to make a multi-passage capillary assembly by using hollow tubes instead of fibers. The circular holes in such a bundle are ordered with no void space in the walls and the assembly can become very large. A variation of this method casts each starting hexagonal member instead of die drawing.
One problem with the current art is that the glass used in image intensifiers is typically a low melting point lead glass. Lead glass can be easily cast or shaped through a die to form the required external hex pattern. However, the low melting point of the lead glass results in a structure that is stiff and easy to shatter. Generally, the higher the melting point of a glass the greater the elasticity. Lead is a very low melting point glass and hence has a low modulus of elasticity and a further problem of chemical erosion. But the biggest disadvantage to this approach is the resultant final shape of the assembly which is a hex. Since the ends of the multi-capillary need to be attached to other parts in a gas chromatograph, the hex shape causes difficulties in getting compression type fittings to interface.
The problem of making connections to capillary structures is not a trivial one. The fine diameters of tubing and the low tensile strength of capillary column materials, such as fused silica, makes the arrangement of capillary columns and of capillary connectors for the capillary tubes especially difficult. The problems are particularly troublesome when connecting the termini of a capillary or, in case of rigid capillary assemblies, the termini of open tubular helical coils to GC instrument conduits. Although many methods and procedures for making such connections are possible the connections generally require bonding to a conduit that has a circular cross-section. Suitable connection arrangements are described in U.S. Pat. No. 5,692,078.
The obvious solution is to make the outer cross section of the multi-capillary a circle for a more compatible fit to conventional compression fittings. This approach though confronts a mathematical problem that nobody has solved and that is: small circles in a larger circle do not pack in a uniform manner. This problem has presented itself in many different forms over the last several hundred years in stranded steel cables, in electrical conduits, etc. Simply put, circles packed together do not want to form an outer shape of a circle--circles packed together with the proper number of elements form hex shaped outlines.
It is an object of this invention to provide a multi-passage capillary assembly that has high ductility and a cross section compatible with the necessary fittings for supplying fluid to the capillary.
It is further object of this invention to provide a multi-passage capillary assembly that provides a high degree of uniformity in the individual cross sections of the multiple capillaries and has an outer cross section of the assembly that is compatible with the necessary fittings for supplying fluid to the capillary.